Data transmission networks, also called telecommunications networks or communications networks, typically include a plurality of units, generally called data terminal equipment (DTE). The latter are also referred to as terminals or stations. A computer connected to such a network is considered to be a terminal. Terminals communicate with each other via a transmission line. An S.sub.0 -type link, which is defined by Recommendation X431 of the CCITT (Consultative Committee on International Telegraphy and Telephony) is a transmission line that includes two pairs of telephone wires, one being dedicated to sending messages and the other being dedicated to receiving messages.
The various terminals of a network send information messages and receive those sent by others. A message is composed of a set of elementary blocks of binary information called frames. Each frame includes a structured array of data including data which defines its beginning and its end, the address of the intended receiving terminal, the address of the sending terminal, the data length, and other useful information.
A current technological trend in the field of data transmission networks is the digital transmission of both voice and data over a common infrastructure. This is due essentially to the progressive introduction of digital techniques into the field of telephony. The integrated service digital network (ISDN) has been introduced in response to this trend.
The ISDN is now used principally in Europe and more particularly in France. A communications link of the S.sub.0 -type, also called an S.sub.0 interface, is one of the standardized links within ISDN. It is used in particular for communications between computers and terminals.
An LS.sub.0 link of this type has a data transfer rate of 288 kbps (144 kbps in each direction, send and receive) and has three separate channels, namely two B-type channels for transmitting data at a rate of 64 kbps and one D-type signalling channel at 16 kbps. The principle of the LS.sub.0 link is time-multiplexing, as defined in the aforementioned Recommendation X431.
A computer typically includes at least one processing unit, an input/output processor, a random access memory, and a read only memory associated with the processor, an input/output controller, all of these elements forming a host system. There are commonly several peripherals cooperative with the host system, such as disk memories or input/output peripherals that facilitate communication of data with a user (such as screen terminals, printers, etc.), each of these peripherals being associated with corresponding peripheral controllers.
All the component parts listed above (aside from the peripherals) are disposed on a set of boards whose dimensions are standardized. These boards are generally connected to one bus of the parallel type which provides communications between the various processors and data transport between the various boards, as well as providing the electrical power supply thereto.
One bus commonly used in present-day practice is called MULTIBUS II, a registered trademark of the Intel Company. The architecture of such a bus is structured around a principal bus of the standardized parallel type according to IEEE 1296 Standard. This principal bus is designated PSB.
As computer networks become more common, the number of computer terminals increases accordingly. This has necessitated the development of programmed communications controllers for reducing the load on the computer's processing unit. Such controllers manage the messages sent by the various terminals to the telecommunications network to which the computer is connected, as well as managing the messages coming from terminals on the network. In current practice, such a communications controller is built around a microprocessor connected to one or more memories, which has a basic program (simpler than that of the central processing unit) containing specialized modules allowing the bus common to the various component elements of the computer and the network transmission line to be managed, and having facilities for constituting message queues (in the memory associated with the microprocessor). This program must also allow a large number of processes to be executed simultaneously and for this purpose must rapidly generate numerous interrupts, which involves high performance mechanisms for changing the microprocessor context, as well as multiple interrupt levels. Such a program, which can be called communications software, is for example the program called CNS used in the products in the Bull S.A. Company's DN-7XXX series and also in the CNS-A.sub.0 and CNS-A.sub.1 products in the DPS-7000 computer series made by the same company.
In the case where the computer has a MULTIBUS II type bus, the communications processor is connected to it. It is disposed on a board connected to the PSB bus through a coprocessor, for example, of the MPC 82309 type (manufactured by the Intel Company) communicating in message mode with the other functional component parts of the computer.
Since the S.sub.0 -type interface is still relatively new, a communications controller which both connects to a Multibus II and manages one or more of these interfaces is practically nonexistent.